Constant on-time control is widely applied in the area of power supply because of its excellent load transient response, simple circuitry structure and smooth operation mode switching.
FIG. 1 illustrates a constant on-time switch mode power supply 100 as a prior art. The power supply 100 comprises constant on-time signal generator 101, a logic circuit 102, a driver circuit 103, a comparator 104, a feedback circuit 106 and a switch circuit 105. The switch circuit 105 comprises at least a power switch configured to convert an input signal VIN to an output signal VOUT by turning the power switch on and off. The feedback circuit 106 is coupled between an output terminal of the switch circuit 105 and an inverting terminal of the comparator 104, configured to generate a feedback signal VFB to reflect the output signal VOUT. The feedback signal is provided to the inverting terminal of the comparator 104. A non-inverting terminal of the comparator 104 receives a reference signal VREF. The comparator 104 compares the feedback signal VFB with the reference signal VREF. The constant on-time signal generator generates a constant on-time signal to a reset terminal of the logic circuit 102. A set terminal of the logic circuit 102 is coupled to an output terminal of the comparator 104. A control signal is generated according to the constant on-time signal and a comparative result of the comparator 104. The control signal is provided to an input terminal of the driver circuit 103. A driver signal of the driver circuit 103 drives the power switch in switch circuit 105 on and off.
When the switch mode power supply is operating in steady state, the output signal VOUT has a ripple. The control of the power switch in switch circuit 105 depends on the ripple of the output signal VOUT. When the equivalent series resistance (ESR) of an output capacitor in switch circuit 105 is relatively large, the output ripple of the output signal VOUT is also relatively large, which results that the average voltage of the output signal VOUT is larger than a desired value. However, if the ESR of the output capacitor is relatively small, the output ripple of VOUT is also very small. The switch mode power supply is tended to be disturbed, and thereby the output stability is affected.